1. Field of the Invention
The present invention relates to an image processing apparatus which performs tone correction of an input image.
2. Description of the Related Arts
As a conventional tone correction technique, the following technique has been proposed. That is, mountain-shaped distributions are detected from a histogram, a feature amount of a dark part is calculated by calculating, for example, an average luminance of the mountain-shaped distributions at specific positions, and the luminance tone of the dark part is corrected by applying correction for raising that feature amount to a certain target value. Japanese Patent No. 3772133 (to be referred to as JPB3772133 hereinafter) describes an arrangement which calculates a boundary value between a low-luminance part and high-luminance part from mountain-shaped distributions of a histogram, and calculates the barycenter of the low-luminance part as a feature amount of a dark part. Also, Japanese Patent No. 4018524 (to be referred to as JPB4018524 hereinafter) describes an arrangement which calculates, for example, an average luminance of a second brightest mountain-shaped distribution of those which form a histogram as a feature amount of a dark part.
Since the shape of a histogram finely varies depending on objects included in an image, mountain-shaped distributions to be originally extracted cannot often be extracted depending on the shape of the histogram in JPB3772133. JPB4018524 copes with this problem, and adopts a method of smoothing a histogram and coupling a plurality of mountains using a threshold. However, the conventional methods suffer the following problems since they divide a histogram into a low-luminance part and high-luminance part using a threshold (boundary value) or specify a mountain-shaped distribution in which an average luminance value is to be calculated.
That is, with the method using a threshold, even when similar scenes are sensed, different boundary values may be acquired or whether or not to couple mountains may be different depending on slight differences of histograms. In this case, although similar scenes are sensed, tone correction results may be largely different.